The invention relates to a process for the continuous production of particleboards or fiberboards, in which a particle mat of a mixture of large-area oriented wood particles and a phenolic resin binder is scattered onto a conveyor belt and pressed between heated press platens of a continuously operating press using pressure and heat to form a particleboard or fiberboard broadly, a particleboard/fiberboard.
The invention furthermore relates to a plant for carrying out the process of the invention.
The pressing factor in the processing of such OS (oriented strand) boards is about twice as high as in normal particleboard production. For this reason, the production of OS boards was only economic on multiplaten plants having a very high number of platens. For the same reason, the use of continuously operating presses has hitherto not become established, because, as a result of the very high pressing factor, excessively long presses would have to be used which would mean excessively high capital investment in comparison with productivity. Despite the limitations, the ready-built house industry in particular demands both particleboards having a smooth surface and particleboards in which at least one side has a surface structure in the form of a screen impression of a woven or knitted metal wire screen.
The woven metal wire screen here has two functions: (i) to transport the coarse wood chips scattered onto the woven metal wire belt, which chips cannot be precompacted, for example in continuous double-band presses, sufficiently for further transport at belt transitions to be possible, and (ii) to provide a surface structure on the pressed OS boards which are functionally designed for later further processing, for example in the ready-built house industry.
The high pressing factor required in the processing of such OS boards is caused by the deleterious influence of the coarse particle structure and by the following:
the processing of all wood-based boards, such as particleboards, MDF (medium density fiber) boards or OS boards, is carried out technologically according to the principles that the wood particles, in this case the large-area oriented particles for the OS boards, are wetted by moist, liquid resin components (for example, phenolic resin binders) and, as a result of the presence of water, the heating of the particle mat in the press causes this water to vaporize and, as a result of the steam formation, in particular in the core of the boards to be produced, a temperature environment is produced which is .gtoreq.100.degree. C.
In the normal production of particleboards or MDF boards, the particle mat is enclosed by smooth press surfaces (wood plates or steel bands), and a pressure greater than 1 bar can form between the large-area pressing zones. According to the vapor pressure graph, the temperature increases with increasing steam pressure and in general, a temperature level of about 120.degree. C. is established in the core of the boards between the upper and lower press surfaces. The steam pressures of greater than 1 bar result in an accelerated steam transfer from the outer layers to the middle layers, which causes accelerated curing, particularly in the core of the boards.
In the production of OS boards, as a result of the woven metal wire belt, this increased steam pressure cannot become established because the woven belt does not allow a buildup of pressure, so that there is formation of only wet steam in the range around 100.degree. C. As a result, accelerated curing in the core of the board is not possible. This leads in the end to the increased requirements of pressing factors to about twice as high as in the case of normal particleboard manufacture.
To further limit the buildup of steam temperature and pressure, the woven metal wire belt and the particle mat have to be heated in the heated pressing zones from a transport temperature of from about 20.degree. to 40.degree. C. to the production temperature in the heated pressing zones. This too causes a reduction in the pressing factor. Furthermore, the woven metal wire belt, at least on the woven metal side, results in poorer heat transfer from the bottom heating plate to the material being pressed.
DE-C 41 37 845 discloses an improvement of the pressing factor in multiplaten presses by arranging a surface structure-forming screen having a woven metal wire belt or knitted metal wire mesh between at least one of the press platens and the particle mat, which screen has a circumferential sealing edge strip, and by orienting the mat to be pressed and the sealing edge strip in relation to one another in such a way that the sealing edge strip is flush with the particle mat or the particle mat projects by some centimeters in its edge region. The sealing edge strip is arranged in such a way that, during the pressing process, it largely or completely prevents the escape of liberated steam. This leads to the mat subjected to the pressing process having a very much more homogeneous temperature distribution than when carrying out the pressing process using a screen without such an edge seal. However, this disclosure does not suggest how the pressing factor can be improved when pressing OS boards in a continuously operating press. Rather, there is still the view among those skilled in the art that large-area, oriented wood particles cannot be economically processed in continuously operating presses to give particleboards.